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June 2016 Archives

This movie is a demonstration of laminar flow. My colleague Julia Mullarney used it last week in our Osborne lectures to high-school students to demonstrate what turbulent flow ISN'T. Basically, laminar flow is time-reversal invariant. This implies a few things, but, notably here that if you reverse the processes involved you get back to where you started with. This is the problem that micro-organisms face when they move. Any motion that has time-reversal symmetry (like a swimmer kicking their legs, or a scallop shell opening and closing) will get them nowhere. The solution for the micro-organism is to rotate a flagellum (or two). A rotation breaks time-reversal symmetry, since a clockwise rotation does not look the same as an anticlockwise one. More of that is discussed here.

In Julia's case, however, she is interested in turbulent flow. Turbulence is characterised by energy loss (hence the fact that you can't get back to where you started) and structure on many length scales. It has been labelled as one of physics' most stubborn problems.

When I meet God, I am going to ask him two questions: Why relativity? And why turbulence? I really believe he will have an answer for the first. - Werner Heisenberg. Maybe. Or Horace Lamb...

She gave a very entertaining talk about her work on turbulence, in amongst the monster mangroves of the Mekong delta in southern Vietnam. The mangroves play an important role in shaping the costal environment, and their effect on turbulence is significant. They have the role (if I understood Julia correctly) of both aiding deposition of sediment (by calming the flow of incoming waves) but also encouraging its loss (by inducing turbulence at small scales leading to scouring of sediment around mangrove plants). Only by measuring the flow on a fine length scale, can these effects be looked at in detail. Really exciting stuff.